Sensor signal output circuit

ABSTRACT

A sensor signal output circuit includes a first differential amplifier, a first load resistor, a first transistor, a second transistor and a limiter section. The limiter section includes at least a second differential amplifier, which includes an input end coupled to output terminal and an other input end coupled to second reference voltage setting part, a second load resistor for a second differential amplifier, and a third transistor, which includes a gate connected to an output end of the second differential amplifier and a source connected to a gate of the second transistor.

THIS APPLICATION IS A U.S. NATIONAL PHASE APPLICATION OF PCTINTERNATIONAL APPLICATION PCT/JP03/009791.

TECHNICAL FIELD

The present invention relates to a sensor signal output circuit used,for example, in a sensor which detects acceleration, angular velocity,pressure or the like.

BACKGROUND ART

A sensor for detecting acceleration, angular velocity, pressure or thelike generally includes a transducing element for converting the amountof displacement of an object to be detected into an electric signal, anda circuit which electrically amplifies the weak electric signal outputfrom the element for output. The known sensor signal output circuit isillustrated by FIG. 7.

In FIG. 7, first differential amplifier 26 is formed of transistors 1, 2having their respective sources connected to each other, and constantcurrent source 20 connected between the sources of transistors 1, 2 andfirst power supply terminal 33. The electric signal from the sensor isinput to a gate of transistor 1, and first reference voltage settingpart 28 is provided at a gate of transistor 2.

First load resistor 30 is an active load for first differentialamplifier 26. This load resistor 30 is formed of diode-connectedtransistor 3, and transistor 4 having its gate connected to a gate oftransistor 3. The gate and a drain of transistor 3 are connected to adrain of transistor 1, while a source of this transistor 3 is connectedto second power supply terminal 34. Transistor 4 has a drain connectedto a drain of transistor 2, and a source connected to second powersupply terminal 34.

First transistor 5 for preamplification has a gate connected to thedrain of transistor 2, a source coupled to second power supply terminal34 via first constant current source 21, and a drain connected to firstpower supply terminal 33. The drain of transistor 2 provides an outputof first differential amplifier 26.

Second transistor 6 for output has a gate connected to the source offirst transistor 5, a source connected to second power supply terminal34 and a drain coupled to first power supply terminal 33 via secondconstant current source 22. The drain of second transistor 6 is alsoconnected to output terminal 32.

In this sensor signal output circuit, the sum of current flowing fromthe drain of transistor 3 to the drain of transistor 1 and currentflowing from the drain of transistor 4 to the drain of transistor 2 ismaintained, so that as the signal input to the gate of transistor 1increases, drain voltage of transistor 2 or the output of firstdifferential amplifier 26 increases accordingly. Since the gate of firsttransistor 5 is at the same potential as the drain of transistor 2,source voltage of first transistor 5 increases. This increase in thesource voltage of first transistor 5 results in a decrease in drainvoltage of second transistor 6, whereby output voltage of outputterminal 32 decreases.

When a break in a wire or a short circuit to first power supply terminal33 occurs in transmission of the output voltage to a receiving circuitthrough use of the wire, the following problem occurs.

Although the voltage input to the receiving circuit is equal to apotential of first power supply terminal 33, with only this outputcircuit, a determination cannot be made as to whether this input voltageis a normal output of the sensor or a voltage resulting from the breakin the wire or the short circuit to first power supply terminal 33.Accordingly, another circuit has been required for detecting the shortcircuit.

DISCLOSURE OF THE INVENTION

A sensor signal output circuit includes:

a first differential amplifier including an input end connected to afirst reference voltage setting part, and an other input end to which asignal is input;

a first load resistor for the first differential amplifier;

a first transistor including a gate connected to an output end of thefirst differential amplifier, and a source connected to a first constantcurrent source;

a second transistor including a gate connected to a point of connectionbetween the first constant current source and the first transistor, anda drain connected to a second constant current source;

an output terminal connected to the drain of the second transistor; and

a limiter section, the limiter section including at least:

-   -   a second differential amplifier including an input end coupled        to the output terminal, and an other input end coupled to a        second reference voltage setting part;    -   a second load resistor for the second differential amplifier;        and    -   a third transistor including a gate connected to an output end        of the second differential amplifier, and a source connected to        the gate of the second transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a sensor signal output circuit in accordance witha first exemplary embodiment of the present invention.

FIG. 2 is a diagram of a sensor signal output circuit in accordance witha second exemplary embodiment of the present invention.

FIG. 3 is a diagram of a sensor signal output circuit in accordance withthe second embodiment of the invention.

FIG. 4 is a diagram of a sensor signal output circuit in accordance withthe second embodiment of the invention.

FIG. 5 is a diagram of a sensor signal output circuit in accordance withthe second embodiment of the invention.

FIG. 6 is a diagram of a sensor signal output circuit in accordance withthe second embodiment of the invention.

FIG. 7 is a diagram of a conventional sensor signal output circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Exemplary embodiments of a sensor signal output circuit according to thepresent invention are demonstrated hereinafter with reference to theaccompanying drawings. It is to be noted that a sensor is used as meansfor inputting a signal to the sensor signal output circuit.

(First Exemplary Embodiment)

The first exemplary embodiment is described with reference to FIG. 1.

First, a description is provided of the structure of a sensor signaloutput circuit of the present embodiment.

In FIG. 1, first differential amplifier 26 is formed of transistors 1, 2having their respective sources connected to each other, and constantcurrent source 20 connected between the common sources of transistors 1,2 and first power supply terminal 33. A signal from a sensor is input toa gate of transistor 1, and first reference voltage setting part 28provides a first reference voltage to a gate of transistor 2.

First load resistor 30 is an active load for first differentialamplifier 26. This load resistor 30 is formed of diode-connectedtransistor 3, and transistor 4 having its gate connected to a gate oftransistor 3. The gate and a drain of transistor 3 are connected to adrain of transistor 1, while a source of this transistor 3 is connectedto second power supply terminal 34. Transistor 4 has a drain connectedto a drain of transistor 2, and a source connected to second powersupply terminal 34.

First transistor 5 for preamplification has a gate connected to thedrain of transistor 2, a source coupled to second power supply terminal34 via first constant current source 21, and a drain connected to firstpower supply terminal 33. The drain of transistor 2 provides an outputof first differential amplifier 26.

Second transistor 6 for output has a gate connected to the source offirst transistor 5, a source connected to second power supply terminal34, and a drain connected to output terminal 32 and coupled to firstpower supply terminal 33 via second constant current source 22.

Second differential amplifier 27 is formed of sixth and seventhtransistors 8, 9 having their respective sources connected to eachother, and third constant current source 23 connected between thesources of sixth and seventh transistors 8, 9 and first power supplyterminal 33. A gate of sixth transistor 8 is coupled to second powersupply terminal 34 via constant current source 24 and connected to asource of output monitoring transistor 12. Transistor 12 has a drainconnected to first power supply terminal 33, and a gate connected tooutput terminal 32.

A gate of seventh transistor 9 is coupled to second power supplyterminal 34 via constant current source 25 and connected to a source offourth transistor 13 of second reference voltage setting part 38. Fourthtransistor 13 has a drain connected to first power supply terminal 33,and a gate connected to second reference voltage 29, which is a limitvoltage.

Second load resistor 31 is an active load for second differentialamplifier 27. This second load resistor 31 is formed of diode-connectedtransistor 14, and transistor 15 having its gate connected to a gate oftransistor 14. The gate and a drain of transistor 14 are connected to adrain of sixth transistor 8, while a source of this transistor 14 isconnected to second power supply terminal 34. Transistor 15 has a drainconnected to a drain of seventh transistor 9, and a source connected tosecond power supply terminal 34.

Output limiting third transistor 7 has a gate connected to the drain ofseventh transistor 9, a source connected to the source of firsttransistor 5, the gate of second transistor 6 and first constant currentsource 21, and a drain connected to first power supply terminal 33. Thedrain of seventh transistor 9 provides an output of second differentialamplifier 27.

A limiter section is formed of at least third transistor 7, seconddifferential amplifier 27 and second reference voltage setting part 38.

A description is provided next of the operation of the sensor signaloutput circuit having the above structure.

The sum of current flowing from the drain of transistor 3 to the drainof transistor 1 and current flowing from the drain of transistor 4 tothe drain of transistor 2 is maintained, so that as the signal input tothe gate of transistor 1 increases, drain voltage of transistor 2 or theoutput of first differential amplifier 26 increases accordingly. Sincethe drain of transistor 2 and the gate of first transistor 5 are at thesame potential, source voltage of first transistor 5 increases, anddrain voltage of second transistor 6 decreases. Consequently, outputvoltage of output terminal 32 decreases.

If the output voltage is lower than second reference voltage 29 or thelimit voltage, the decrease in the output voltage results in a decreasein gate voltage of transistor 12, and in synchronization with the gatevoltage of transistor 12, gate voltage of sixth transistor 8 decreases.

Second reference voltage 29 which is the limit voltage applied to thegate of fourth transistor 13 is applied as a gate voltage of seventhtransistor 9.

For this reason, drain voltage of seventh transistor 9 or the output ofsecond differential amplifier 27 and gate voltage of third transistor 7decrease. In synchronization with the gate voltage of transistor 7,source voltage of transistor 7 and gate voltage of second transistor 6decrease, whereby the drain voltage of second transistor 6 increases tosecond reference voltage 29 or the limit voltage.

In this way, the output voltage does not fall short of second referencevoltage 29, but is maintained at second reference voltage 29 even whenthe input voltage such as to cause the output voltage to fall short ofsecond reference voltage 29 is input. It is to be noted here that firstand third transistors 5, 7 have their respective sources connected toeach other and their respective drains connected to each other, thushaving the common source voltage, so that third transistor 7 is turnedon, while first transistor 5 is turned off. The output of firstdifferential amplifier 26 is thus cut off, not affecting the outputvoltage.

If the output voltage is higher than second reference voltage 29 or thelimit voltage, third transistor 7 is turned off, and first transistor 5is turned on because these transistors 5, 7 have the common sourcevoltage. The output of second differential amplifier 27 is thus cut off,not affecting the output voltage.

As described above, the output voltage of output terminal 32 does notfall short of second reference voltage 29, so that setting this secondreference voltage 29 higher than a potential of first power supplyterminal 33 eliminates the possibility that the output voltage will beequal to the potential of first power supply terminal 33 under normalconditions. This allows a receiving circuit to judge that a break in awire or a short circuit to first power supply terminal 33 has occurredwhen the output voltage has become equal to the potential of first powersupply terminal 33.

The supplied limit voltage effected by the operation of the limitersection for the output voltage is accurate and stable with respect totemperature or the like and can be changed easily by changing secondreference voltage 29.

In the sensor signal output circuit of the present invention, secondreference voltage setting part 38 is formed of fourth transistor 13having its gate connected to second reference voltage 29, its drainconnected to first power supply terminal 33 and its source connected tothe input end of second differential amplifier 27, thus advantageouslyfacilitating setting of second reference voltage 29.

In the sensor signal output circuit of this invention, seconddifferential amplifier 27 has a simple structure, which has sixth andseventh transistors 8, 9 having their respective sources connected toeach other, and third constant current source 23 connected to thesources of these transistors 8, 9, thus having the advantage of beingcapable of performing an accurate comparison between the output voltageand second reference voltage 29.

Since the output voltage is input directly to second differentialamplifier 27, response is advantageously faster compared with caseswhere the output voltage is input by way of another circuit or the like.

(Second Exemplary Embodiment)

Referring to FIGS. 2-6, a description is provided hereinafter of thesecond exemplary embodiment.

FIG. 2 is a diagram of a sensor signal output circuit in accordance withthe present embodiment. Elements similar to those in the firstembodiment have the same reference marks, and the descriptions of thoseelements are omitted.

The second embodiment differs from the first embodiment in that thepresent embodiment includes limiter disengagement part 35 and outputsaturation part 36.

Limiter disengagement part 35 is formed of fifth transistor 16 connectedin parallel to fourth transistor 13. Output saturation part 36 is formedof tenth transistor 17 provided between first reference voltage settingpart 28 and first power supply terminal 33. The operation of limiterdisengagement part 35 and the operation of output saturation part 36 arecontrolled by abnormality detector 37.

Abnormality detector 37 produces a control signal when, for example, asensor experiences excessive or abnormal disturbance (such as vibrationor an electromagnetic wave) during a period from when the sensor startsto when an output reaches a stable region.

A description is provided next of the operation of the sensor signaloutput circuit of the present embodiment.

If the abnormality detection signal is input from abnormality detector37 to a gate of fifth transistor 16 when the output voltage is about tobecome lower than second reference voltage 29 or a limit voltage as aresult of an increase in voltage input to a gate of transistor 1, fifthtransistor 16 forming limiter disengagement part 35 is turned on, thusestablishing a short circuit between a source and a drain of fourthtransistor 13. Consequently, voltage between a gate and the source offourth transistor 13 and second reference voltage 29 are eliminated.Gate voltage of seventh transistor 9 is thus adjusted to a level lowerthan level-adjusted voltage of sixth transistor 8, decreasing by thevoltage between the gate and source of transistor 13 and secondreference voltage 29. This means that apparent reference voltage hasdecreased by the voltage between the gate and source of transistor 13and second reference voltage 29, so that the output voltage isconditioned to always become higher than second reference voltage 29 orthe limit voltage. Here, drain voltage of seventh transistor 9 or anoutput of second differential amplifier 27 and gate voltage of thirdtransistor 7 increase. Since first and third transistors 5, 7 havecommon source voltage, third transistor 7 is turned off, while firsttransistor 5 is turned on, whereby the output of second differentialamplifier 27 is cut off, not affecting the output voltage. Consequently,the output voltage responsive to the input can be output even in aregion lower than second reference voltage 29, that is, even when theinput voltage such as to cause the output voltage to fall short ofsecond reference voltage 29 is input.

Limiter disengagement part 35 can have another structure, which is asfollows. As shown in FIG. 3, eighth transistor 18 is provided. Thistransistor 18 has a source connected to common sources of sixth andseventh transistors 8, 9, a drain connected to second power supplyterminal 34 and a gate connected to an output end of abnormalitydetector 37.

Limiter disengagement part 35 can have still another structure such asshown in FIG. 4. In other words, ninth transistor 19 is provided. Thistransistor 19 has a source connected to a gate of third transistor 7, adrain connected to second power supply terminal 34, and a gate connectedto the output end of abnormality detector 37. In either of these FIGS. 3and 4, first and third transistors 5, 7 have common source voltage as inFIG. 2, so that third transistor 7 is turned off, while first transistor5 is turned on, whereby the output of second differential amplifier 27is cut off, not affecting the output voltage. Consequently, the outputvoltage responsive to the input can be output even in a region lowerthan second reference voltage 29, that is, even when the input voltagesuch as to cause the output voltage to fall short of second referencevoltage 29 is input.

The sensor signal output circuit thus has the advantage of not making amisjudgment as a result of receiving the signal, which is output by themeans for inputting the signal to the sensor signal output circuit when,for example, the system is not stable right after power-up or the like.

If abnormality detector 37 provides the abnormality detection output toa gate of tenth transistor 17 of output saturation part 36 and the gateof fifth transistor 16 of limiter disengagement part 35 at the sametime, a limiter section is disengaged in the manner described above, andtenth transistor 17 is turned on. Since a gate of transistor 2 isgrounded to first power supply terminal 33, the voltage input to thegate of transistor 1 always becomes higher than gate voltage oftransistor 2. For this reason, drain voltage of transistor 2 or anoutput of first differential amplifier 26 and base voltage of firsttransistor 5, the gate of which is at the same potential as a drain oftransistor 2, increase, and the source voltage of transistor 5 alsoincreases. Accordingly, drain voltage of second transistor 6 decreases,whereby the output voltage of output terminal 32 decreases to less thanthe gate voltage of transistor 2, thus becoming equal to a potential offirst power supply terminal 33.

With the above-described structure, the output similar to a voltageresulting from a break in a wire or a short circuit to first powersupply terminal 33 can be achieved when the abnormality is detected, sothat the information about the detected abnormality can be transmittedto a receiving circuit via signal output terminal 32 without anotherterminal provided for abnormality detection.

In the embodiment, the sensor signal output circuit is not provided witha level adjustment part. However, the level adjustment part may beprovided on an as needed basis. For example, diode-connected transistors10, 11 may be used as shown in FIG. 5. In this case, transistor 10 isinterposed between sixth transistor 8 and transistor 12, whiletransistor 11 is interposed between seventh transistor 9 and fourthtransistor 13.

Even the use of bipolar transistors (not shown) instead of thetransistors used in the embodiment affords the same advantage. In such acase, the source corresponds to an emitter, the drain corresponds to acollector, and the gate corresponds to a base.

In cases where all the transistors of FIG. 1 each have P replaced with Nand N replaced with P as shown in FIG. 6, an upper limit can be set onthe output voltage. In other words, while a limit voltage, that is, alower limit voltage has been set by second reference voltage settingpart 38 in the sensor signal output circuit of the present invention,the upper limit voltage can be set in a similar manner. This provides anadvantage that a short circuit to second power supply terminal 34 can bedetected without a detection circuit. Even the use of bipolartransistors such as described above provides the same advantage.

In the sensor signal output circuit of the present invention, limiterdisengagement part 35 is provided to the limiter section, thus providingthe same advantage.

In the sensor signal output circuit of this invention, limiterdisengagement part 35 is formed of fifth transistor 16 in parallel withfourth transistor 13 of second reference voltage setting part 38, andthis fifth transistor 16 has its drain connected to the source of fourthtransistor 13, its source connected to first power supply terminal 33,and its gate serving as an input end for the limiter disengagementsignal, thus providing the same advantage.

In another sensor signal output circuit of this invention, limiterdisengagement part 35 is formed of eighth transistor 18 having thesource connected to the common sources of sixth and seventh transistors8, 9 of second differential amplifier 27, the drain connected to secondpower supply terminal 34, and the gate serving as an input end for thelimiter disengagement signal, thus providing the same advantage.

In still another sensor signal output circuit of this invention, limiterdisengagement part 35 is formed of ninth transistor 19 having the sourceconnected to the gate of third transistor 7, the drain connected tosecond power supply terminal 34, and the gate serving as an input endfor the limiter disengagement signal, thus providing the same advantage.

The sensor signal output circuit of this invention is provided withabnormality detector 37 for operating limiter disengagement part 35 inabnormality, and output saturation part 36 for holding the outputvoltage at a voltage lower than second reference voltage 29 upon receiptof the abnormality detection signal output from abnormality detector 37,thus providing the advantage that the abnormality other than the breakin the wire or the short circuit to first power supply terminal 33 canbe detected.

In the sensor signal output circuit of this invention, output saturationpart 36 is formed of tenth transistor 17 having its drain connected tothe input end (to which the first reference voltage is input) of firstdifferential amplifier 26, its source connected to the potential offirst power supply terminal 33, and its gate connected to abnormalitydetector 37, thus providing the same advantage.

In each of the foregoing embodiments, the sensor is used as the meansfor inputting the signal to the sensor signal output circuit. However,such means is not limited to the sensor.

As described above, the present invention has the advantage that thebreak in the wire or the short circuit to first power supply terminal 33can be detected without the detection circuit.

In other words, since the output voltage of the output terminal does notfall short of second reference voltage 29, setting this referencevoltage 29 higher than the potential of first power supply terminal 33eliminates the possibility that the output voltage will be equal to thepotential of first power supply terminal 33 under normal conditions.Thus, the judgment can be made that the break in the wire or the shortcircuit to first power supply terminal 33 has occurred when the outputvoltage has become equal to the potential of first power supply terminal33.

INDUSTRIAL APPLICABILITY

The present invention relates to a sensor signal output circuit used,for example, in a sensor which detects acceleration, angular velocity,pressure or the like. This invention allows detection of a break in awire or a short circuit to a power supply terminal without a detectioncircuit.

1. A sensor signal output circuit comprising: a first differentialamplifier including an input end connected to a first reference voltagesetting part, and an other input end to which a signal is input; a firstload resistor for the first differential amplifier; a first transistorincluding a gate connected to an output end of the first differentialamplifier, and a source connected to a first constant current source; asecond transistor including a gate connected to a point of connectionbetween the first constant current source and the first transistor, anda drain connected to a second constant current source; an outputterminal connected to the drain of the second transistor; and a limitersection, the limiter section including at least: a second differentialamplifier including an input end coupled to the output terminal, and another input end coupled to a second reference voltage setting part; asecond load resistor for the second differential amplifier; and a thirdtransistor including a gate connected to an output end of the seconddifferential amplifier, and a source connected to the gate of the secondtransistor.
 2. The sensor signal output circuit of claim 1, wherein thelimiter section further includes a limiter disengagement part.
 3. Thesensor signal output circuit of claim 1, wherein the second referencevoltage setting part includes a fourth transistor including a gateconnected to a second reference voltage, a drain connected to a groundpotential, and a source connected to the other input end of the seconddifferential amplifier.
 4. The sensor signal output circuit of claim 1,wherein the second differential amplifier includes a sixth and seventhtransistors including respective sources connected to each other, and athird constant current source connected to the sources of the sixth andseventh transistors.
 5. The sensor signal output circuit of claim 4,wherein the limiter section further includes a limiter disengagementpart.
 6. The sensor signal output circuit of claim 5, wherein thelimiter disengagement part includes a fifth transistor in parallel witha fourth transistor of the second reference voltage setting part, andthe fifth transistor includes a drain connected to a source of thefourth transistor, a source connected to a ground potential, and a gateserving as an input end for a limiter disengagement signal.
 7. Thesensor signal output circuit of claim 5, wherein the limiterdisengagement part includes an eighth transistor including a sourceconnected to the common sources of the sixth and seventh transistors ofthe second differential amplifier, a drain connected to a power supplypotential, and a gate serving as an input end for a limiterdisengagement signal.
 8. The sensor signal output circuit of claim 5,wherein the limiter disengagement part includes a ninth transistorincluding a source connected to the gate of the third transistor, adrain connected to a power supply potential, and a gate serving as aninput end for a limiter disengagement signal.
 9. The sensor signaloutput circuit of claim 5, further comprising: an abnormality detectorfor operating the limiter disengagement part in abnormality; and anoutput saturation part for holding an output voltage at a voltage lowerthan a second reference voltage upon receipt of an abnormality detectionsignal output from the abnormality detector.
 10. The sensor signaloutput circuit of claim 9, wherein the output saturation part includes atenth transistor including a drain connected to the input end of thefirst differential amplifier that is provided with a first referencevoltage, a source connected to a ground potential, and a gate connectedto the abnormality detector.